The present invention relates to a method for operating a Satellite Positioning System (SPS) receiver and more particularly relates to a system in which the receiver provides, through a wireless communication link, information regarding its position.
Conventional Satellite Positioning Systems (SPS) such as the Global Positioning System (GPS) use signals from satellites to determine their position. GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on time of day plus clock timing, which together is herein referred to as ephemeris data. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and furthermore greatly limits battery life in portable operations and applications
Another current limitation of current GPS receivers is that their operation is limited to situations in which multiple satellites are clearly in view, without obstructions, and where a good quality antenna is properly positioned to receive such signals. As such, they are normally unusable in portable body-mounted applications and in areas where there is significant foliage or building blockage and within buildings.
There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites; and (2) computation of the position of the receiving platform using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the time delays measured between the received signal from each satellite and a local clock in the GPS receiver. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. As stated above, collecting this information normally takes a relatively long time (such as thirty seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Recently, GPS receivers have been used with radio transmitters, such as a cellular telephone or a mobile telephone in a car to transmit the position of the receiver as it moves. Conventional combined GPS/communication systems typically transmit a position from the radio transmitter to a remotely located basestation. Typically, the GPS receiver will determine its position and then provide that information to the transmitter which then transmits the determined position before the GPS receiver has determined a next position. This allows an operator at the remotely located basestation which receives, through the radio signal, the position to track the route of the GPS receiver as it moves over time. In an alternative embodiment, described for example in U.S. Pat. No. 5,663,734, the mobile GPS receiver which includes a communication transmitter transmits time-tagged pseudorange information rather than a completed position calculation (such as latitude, longitude, and altitude of the GPS receiver). In this case, the mobile unit, which includes the GPS receiver, will collect GPS signals and processes those signals to determine pseudoranges to the various satellites in view at a particular time and then the transmitter will transmit these pseudoranges to a remotely located basestation which will then process these pseudoranges with the time tags of the pseudoranges plus ephemeris data collected at or supplied to the basestation in order to determine a position of the mobile unit. Also in this case, the transmitter will transmit one set of pseudoranges before the GPS receiver determines a next set of pseudoranges.
While both of these prior approaches provide a way to track the route of a moving GPS receiver, there are several concerns with using these techniques. In the case of the mobile GPS receiver which determines its position and transmits the position to a remotely located basestation, the mobile unit must have a good view of the sky and receive multiple satellites clearly in order to be able to compute the pseudoranges and to read the ephemeris data before the GPS receiver can determine its position. Furthermore, in the case where this mobile GPS receiver attempts to compute several positions and then transmit them in one transmission, this receiver will typically not be able to benefit from differential GPS corrections, unless a large set of differential corrections is buffered at the basestation. A mobile GPS receiver which collects a series of digitized samples of GPS signals and transmits the series in one transmission will consume large amounts of battery power and may cause congestion in the wireless link due to the large amount of data being collected, stored and transmitted. See, for example, European Patent Application 0 508 405.
In the case of the mobile GPS receiver which transmits pseudoranges one at a time, the communication transmitter must be repeatedly powered up in order to transmit each set of pseudoranges after they have been determined. This may tend to decrease battery life in the mobile unit and may also cause congestion in the wireless communication link between the mobile unit and a basestation. Furthermore, the air time costs may be high for such an operation.
Thus it is desirable to provide an improved method and system for providing multiple sets of position information over a period of time through a mobile GPS unit.